AliTaher,
1. If you replace the Quadro FX4800, does the system boot?
2. How many beeps are made when the system refuses to boot? What does the manual say as to that number of beeps with power button flashing red?
3. What was the power supply in the system in which the R7900-6GD5 worked?
I can not find a Passmark benchmark listing for the R7900, R7900-6GD5, or ATI Radeon HD 7990 in any of more than 68,000 Hewlett Packard systems tested. There are two listings in Dell, one is in a Precison T5600 (2X E5-2690), having a 2D of 569 and 3D of 3346. < That is a fairly low score. For comparison, the highest 3D score of a Quadro FX 4800 in a z800 is 1104. The T5600 had a PSU of 635W or 820W.
While the HD 7990 may have a Passmark 3D up to a maximum of 7951, that is in a 4.8GHz system. In the z800 with 2X Xeon X5690, an 8136 mark is achievable using a GTX 970, specified as requiring a minimum 500W power supply. If the problem is power supply limitation, consider a newer series 6 or 8GB GPU, GTX 970 , 980, 980 Ti. Probably, beyond those card, the CPU single-thread performance speed will limit 3D performance.
BambiBoomZ